This invention relates, in general, to manufacturing of semiconductor products, and more particularly, to fabricating masks for lithography.
At present, optical reduction printing of microcircuit patterns dominates the integrated circuit industry. Generally, these microcircuit patterns are originally formed on a photoresist coated glass plate called a master mask or a reticle. An electron beam which is focused into a small spot is scanned over the photoresist covered glass plate, thereby exposing the photoresist on the glass plate and creating a pattern. The glass plate is photographically developed, etched, and used as the master mask to optically transfer the pattern on the master mask to a semiconductor wafer. The transfer of the pattern from the glass plate or the master mask to the semiconductor wafer is achieved typically by a reduction projection tool and ordinary photolithographic techniques. Generally, reduction projection tools reduce the patterns or features on the master mask by 4.times., 5.times., or 10.times. at the surface of the semiconductor wafer.
Resolution of photolithographic techniques is limited by frequency or wavelength of radiation that is used to project the features in the reduction projection tool. Even though optical reduction projection of very small features sizes, such as 0.25 microns or 2,500 angstroms, is achievable, it is a very difficult process to control in a high volume production environment. X-rays, however, are capable of resolving very small features because of their higher frequency or shorter wave length. By using higher frequency or shorter wavelength radiation, such as X-rays, to project small features, control of the lithographic process is greatly simplified.
However, X-ray mask making is a difficult process. X-ray mask making is, generally, achieved by using an electron beam that is focused into an extremely small spot size of approximately 200 or 300 angstroms. An X-ray master mask blank is built from a composite of materials, which is further prepared by coating a surface that is to be photographically exposed with a photoactive material or a photoresist material. The photoresist coated X-ray master mask blank is placed onto a movable precision stage. The stage is then positioned and stopped for the electron beam to expose this one area. The stage is then moved to another area for another exposure. This process is repeated many times until the entire pattern is exposed; however, this process introduces many kinds of errors into the X-ray master mask, such as line width, feature placement, and the like. These errors are then replicated on the semiconductor wafer when the X-ray mask is used to project the pattern onto the semiconductor wafer. Production of X-ray masks by the conventional process is very slow and, therefore time consuming and costly.
Additionally, X-ray master masks deteriorate in time when used in an X-ray projection system. Because the deteriorated masks eventually have to be thrown away, not only causes the cost of operating an X-ray lithography system to increase, but also increases the need for reliable replication of duplicate X-ray masks.
It can be seen that the conventional process of fabricating X-ray master masks is not adequate. The conventional approach causes many kinds of errors and is expensive. Therefore, a method that would substantially improve quality of replication, simplify the process, and reduce the cost is highly desirable.